3 Intruders circle in from offscreen to form a Zelos Force. After a certain time or amount of hits, it shifts forms again into a Brain Golem before disintergrating.
Although probably irrevelant, the acronym O.V.U.M is the spelling of the word Ovum which means egg.
Since Ovum means egg, it means that maybe O.V.U.M. is a renegade Bacterian embryo that uses its cell division abilities to make organisms. When the fight began, it strained itself and then broke down when the fight was over.
It has never been known who exactly O.V.U.M. was and maybe we will never know.
Shortly before the battle begins, you can see a red face which looks more like an Intruder's face. Could this be O.V.U.M's physical form before shapeshifting or did Konami just put that there with no meaning whatsoever?
O.V.U.M.'s name stands for "Original Visions of the Ultimate Monsters", which may mean it is the power of ultimate evil condensed into physical form.
It may also be the fusion of cells from Intruder, Zelos Force, and Brain Golem that ended up in the Dark Nebula and grew into one combined cell.
Of course, "Original Visions of the Ultimate Monsters" is probably just a nickname for Zelos Force who gained the ability to shapeshift. For example, Zelos resembles an ovum cell. Ovum cell, as in O.V.U.M.